Geological surveys to discover natural and manmade structures below the surface of the ground often employ very low frequency acoustic sources and arrays of very low frequency acoustic sensors. Such surveys produce images of underground structures, both man-made and geological, for the purpose of assisting geological and geophysical exploration, such as mineral, oil and natural gas exploration, discovery of underground tunnels, and earthquake fault zone mapping. While suitable for many purposes, such surveys suffer some disadvantages, including that they require large energy sources, involve wide ranging placement of acoustic sources and sensors and the associated costs, and can disrupt animal and human activity.